It happens occasionally in a wet blasting machine, when concentration of the slurry is heavy, or the mass of abrasive particles is heavy, that the blasting operation cannot be performed normally due, for example, to foreign material accidentally mixed in the slurry hindering the slurry flow or clogging the pipe of the slurry line. Such hindrance or clogging interposes a heavy load on the motor of the slurry pump. On the other hand, as wet blasting methods have progressed toward being more and more automated, it is required that the blasting operation always provide consistent and reproducible results. Accordingly, supervision of the slurry flow becomes necessary, because an uneven flow of slurry causes uneven performance and thus nonuniform results.
The assignee of the present invention has a sensing device which can detect an unusual condition of the slurry transportation line in a wet blasting machine. Such device is shown in Japanese provisional publication of Utility patent No. 60-131358 and in FIG. 6 of the present application. As seen in FIG. 6, such device comprises a vibration sensor 1 fixed inside an adapter 2, which adapter 2 is fixed at any desired location on a slurry transportation line up to and including the blasting gun thereof, such a blasting gun being shown at A in appended FIG. 6. The sensor 1 provides an electric output of frequency corresponding to the frequency of vibration to which it is subjected. The output signal of sensor 1 is applied to a series circuit path comprising an amplifier 3, band pass filter 4, analog switch 5 and an indicator 6. The band pass filter 4 eliminates from the electric output signal of the sensor 1 the frequency component representing the vibration frequency received by the vibration sensor 1 due to the normal flow of slurry. The signal outputted by the filter contains the frequency component due to the abnormal slurry line condition, if any.
Considering more specifically the foregoing device, the vibration sensor 1, disposed inside of the adapter 2 which is fixed in some location in the slurry transportation line (including the blasting gun A), detects all vibration in the slurry line and converts it to a corresponding frequency electric signal. The detected frequency electric signal passes through the amplifier 5 to the band pass filter 4, which eliminates the frequency component delivered to the filter when slurry flows normally at the location of the sensor 1. Frequencies other than the eliminated frequency are sent by the filter 4 to the analog switch 5. If the voltage of the filter output frequency signal exceeds a threshold voltage preset in the switch 5 (or the amplitude of the vibration is larger than a predetermined value), the switch 5 sends outputs to the abnormal condition indicator 6 which is thereby activated. The indicator 6 may thus be, for example, a light or audible alarm which is normally turned off but is turned on by said abnormal condition output from switch 5. If the vibration sensor 1 at the nozzle a detects a noise generated by ejection only of compressed air, rather than the noise normally generated by normal flow of the slurry due to normal slurry pump pressure, the abnormal condition indicator 6 of the foregoing device actuates. By such construction, clogging or hindrance in the slurry transportation line is easily detected. On the other hand, the foregoing device disadvantageously has numerous constituent parts.
Also, in wet blasting operations there are very often cases where the operator should be informed if there is clogging in the slurry line before he starts the machine, or if blasting can be done consistently.
The objects and purposes of the present invention include providing a sensing device for detecting unusual conditions in the slurry transportation line of a wet blasting machine which is of more simple construction and which is intended to meet the abovementioned demand for information.